1. Field of the Invention
This invention relates to methods of regulating the growth of plants and, in particular, it relates to methods useful for stimulating the growth and/or fruit production of plants and to compositions useful for regulating plant growth.
2. Description of the Art
Plant growth regulants can be defined as compounds and/or preparations which, in minute amounts, alter the behavior of ornamental and/or crop plants and/or the produce of such plants through physiological (hormonal) rather than physical action. They may either accelerate or retard growth, prolong or break a dormant condition, promote rooting, fruit-set, or increase fruit size or quantity, or affect the growth and/or productivity of plants in other ways. Plant growth regulants are currently classified into one or more of six categories: auxins, gibberellins, cytokinins, ethylene generators, inhibitors, and retardants. Illustrative of known auxins are indole acetic acid, 24-D (2,4-dichlorophenoxyacetic acid), MCPA (4-chloro-2-methyl phenoxyacetic acid), MCPB (4-[(4-chloro-o-tolyl)oxy] butyric acid) which susceptible plants oxidize to MCPA, and BNOA (beta-napthoxyacetic acid). Gibberellins include gibberellic acid and its derivatives, while cytokinins included compositions such as zeatin, kinentin, and benzyl anidene. Presently known ethylene generators include ethylene and Ethephon [(2-chloroethyl) phosphoric acid]. Presently known inhibitors include benezoic acid, gallic acid, and cinnamic acid, while retardants, a recently developed class of plant growth regulants, include compositions which are especially useful in plant height control, particularly in commercial greenhouse-grown floricultural crops.
Lactic acid (alpha-hydroxypropionic acid) is well known and is widely employed in industry as a chemical intermediate. It is usually present in the form of the racemic mixture which is an equimolar mixture of the two possible optical isomers of alpha-hydroxypropionic acid - the levorotatory and dextrorotatory isomers. Levorotatory (1) isomers are isomers of an optically active compound which rotate a beam of polarized light to the left; the dextrorotatory (d) are isomers of the same compound which rotate a beam of polarized light to the right. A second convention employed to define the configurational relationships of dissimilar functional groups bonded to an asymmetric carbon atom, the Fischer method, is based on the geometric arrangement of functional groups relative to each other rather than on the direction (left or right) in which a standard solution of the compound rotates a beam of polarized light. In accordance with the Fischer method, any compound which contains an asymmetric carbon atom of the same configuration as the asymmetric carbon in the arbitrary standard dextrorotatory glyceraldehyde is classified in the D series, while compounds in which the asymmetric carbon atom has the opposite configuration are classified in the L series. Although the Fischer D and L classifications do not correlate with dextro- (d) and levorotatory 1 optical activity for all compounds, those classifications can be used in combination with the optical activity classifications d and 1 to define both the geometric arrangement and specific optical activity of any optically active isomer. Thus, the L-isomer of lactic acid, which is dextrorotatory, is defined as L-(d)-lactic acid, and the D isomer is defined as D-(1)-lactic acid. However, both of these characteristics of relatively simple compounds, such as lactic acid, can be adequately defined by reference to only one classification system. L-lactic acid is known to be dextrorotatory, and 1-lactic acid is known to have the D configuration according to Fischer. For this reason, the D and L isomers of lactic acid are usually identified only by the D and L designations and without explicit reference to their optical activity. The Fischer classification method is well known in the art and is discussed in more detail in "Introduction to Organic Chemistry", Fieser and Fieser, D. C. Heath and Co., Boston, Mass., (1957) at pages 209-215.
Lactic acid is prevalent in a variety of synthetic and naturally occurring products such as dairy products and fermentation products in which it occurs primarily as the racemic mixture. Specialized fermentation processes can be employed to selectively manufacture either the levorotatory or dextrorotatory isomers. Although some commercially available agricultural products contain fermentation products and lactic acid and are marketed for various applications in the agricultural industry, it has not been observed or suggested that L-(d)-lactic acid is an active plant growth regulant. Furthermore, the lactic acid-containing compositions which are marketed in the agricultural industry usually contain the racemic mixture of both optical isomers in addition to cations such as sodium, potassium, ammonium, etc., and/or other compounds such as surfactants, pesticides, etc., which can react with L-lactic acid and destroy its growth regulant activity.
It also has been suggested that alpha-hydroxy carboxylic acids of higher molecular weight than lactic acid exhibit specific growth regulant activity regardless of the configuration or optical activity of the carboxylic acid employed. U.S. Pat. No. 3,712,804, Mueller et al., discloses that certain alpha-substituted carboxylic acids increase the yield of certain crops by improving the ability of the plant to assimilate water from its environment. The acids have 7 to 10 carbon atoms per molecule and the alpha carbon atom is substituted with one or more functional groups including oxy, hydroxy, amine, and carboxyl groups. The acids are applied to very young plants and the salts and lower alkyl esters and amines have growth regulant activity similar to that of the free acid. The compositions can also contain wetting agents.
The plant growth regulants referred to above and otherwise known in the art, including those discussed in U.S. Pat. No. 3,712,804, all suffer from certain disadvantages that make their use, at least in some applications, less desirable than would be the use of L-lactic acid. Many growth regulant compositions, particularly those which exhibit herbicidal activity at higher dosage rates, are toxic to plants, the environment, and/or animals, including humans. Many are not readily available and are relatively expensive to manufacture as compared to L-lactic acid. Also, many of the known growth regulants, such as the alpha-functional carboxylic acids, salts, esters and amines discussed in U.S. Pat. No. 3,712,804, require plant treatment at a time that may not be opportune for the grower in all instances. Furthermore, many known regulants exhibit a limited spectrum of growth regulant activity, are not useful with many plant varieties, and/or do not adequately regulate crop productivity.
Accordingly, a need exists for improved methods for regulating the growth of plants and for improved compositions useful in such methods. In particular, a need exists for improved methods and compositions for stimulating the desired growth of plants, reducing the toxic effects of such methods and compositions on the environment and animals, including humans, and reducing the expense of so regulating plant growth.